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Long aluminum box

12-17-2013, 08:41 AM

I've got a need for a long ally box. About 95" x 40" x 16", a long, low box probably with 22.5" long gullwing lift up lids at each end and a ~40" central section. I was thinking about using 3/32" 5052. It's gotta fit sails, for a boat trailer, and assorted gear and be lockable to leave it overnight reasonably securely. All the premade boxes that people are selling around here are nowhere near this long, which makes me think that there might be structural issues I'm not aware of. Is this a feasible design? Is there a reason no-one makes premade boxes this long? Would I need to think about some internal bracing using square tube or something?

That box is going weigh around 250 pounds empty. I would think that it would be plenty strong depending on how you mount it to the trailer. I don't know how much flex is in the trailer either. You may need to mount it with polyurethane body mounts if you have to deal with trailer flex.

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Most boxes are not made to big for either handling purposes or there isn't a market. Cross bed pickup tool boxes are that size because of what they fit. The box you describe would be an odd size imo.

As far as structural issues it depends on how it's mounted & how much weight it needs to hold. If the floor of the box will be laying on the deck of a trailer then it really won't be an issue. If the floor will have no support, like a pick up box, then that has to be figured into the design. What does everything weigh & will it be in there while bouncing down the road?

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It will be up high on top of a hoop frame. The frame is made out of 3/4" steel pipe with wall thickness just over 3/32". Where the box will go there will be 4 cross bars to bolt down onto spanning most of the length of the box. I was thinking about a thin layer of rubber to prevent galvanic corrosion.

Stuff to go in the box is just sails, tools, some rigging equipment and spares. Maybe 20-30lb. It otherwise fits in the car but the trailer box would mean it can be out of the way and can be semi-securely left overnight if we leave the trailer somewhere where there's a sailing event.

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Yeah, I thought of that. But here's where I make my dumbass confession - I have no experience whatsoever with aluminum, so I thought 3/32" would increase my chances of success. I can save 25% of the cost by moving to 5/64", or almost half the cost by using 1/16". But I was concerned about blowing holes. It's not something I want to experiment with. I don't want to buy a sheet and chop it up and see how it goes before committing, that'll be wasted money.

My (considerable, but only as a hobbyist) experience is all with steel, and 3/32" is a lot more forgiving than 1/16". I'm wondering if I commit to 1/16" and practice with offcuts (there will be some) and use backing bars, whether I can expect success. I will be using a Dynasty 200 DX, so it won't be equipment holding me back. I'll need 4 sheets of 8' x 4', and each sheet will have leftover so I can cut it into scrap for practice.

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If you use angle or tube behind the seams you'll be welding on thicker joints. Lap joint welds on the inside will also be rather simple but will increase the odds of burn through. Just need to make sure you clean the work well with a flap wheel prior to welding. Any oxide layer will slow the puddles formation while still heating the work to where it'll sag or even drip.

Check with your yard to see if you can get longer plate too. I'm pretty sure you can get diamond plate in 12' from the mill.

I'd still use the heavier stuff for the lids. That'd mean less work and easier designing.

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I would be sure to get many scraps to practice on, as i bet u will be blowing holes in it, also i would try to have it bent to avoid as much welding as possible. Thin aluminum butt joints are not easy, i would be sure to use a backing plate. Also you will be moving very fast at the end of the weld.
Kevin